1. Field of the Invention
The present invention relates to an improvement of the focal plane shutter device for a camera structured by a divisional blade type shutter.
2. Related Background Art
Traditionally, there have been proposed devices having various structures as focal plane shutter devices for a camera. As one of them, the so-called vertical movement type focal plane shutter device is known, in which the front and rear shutters formed with the blade groups each having a plurality of divided blades are sequentially driven to travel with a time difference on the photographing image plane with an aperture, so as to expos to the film plane through a slit produced between the blade groups.
For the focal plane shutter device mentioned above, it is necessary to prevent the camera body from being vibrated due to the reaction of the movement of the aforesaid shutter when the shutter is actuated to drive the front and rear blade groups. This is needed to avoid the so-called camera blurring. The causes of the vibration given to the camera body include those accompanying the mirror up operation and the like and the shock generated at the time of the shutter movement termination. Particularly, the latter, which is the shock following the shutter actuation, takes place during the exposure and produces an adverse effect as camera blurring on a picture to be taken. It is therefore desirable to soften or reduce this shock resulting from the shutter actuation as much as possible.
For this purpose, various preventive measures have been proposed. There is known a structure such as disclosed in U.S. Pat, No. 4,458,998, in which at least in a part of one of the driving systems for the two shading members, front and rear, formed by the shutter blinds or shutter blades, a balancer is movably provided as a balancing weight to offset the inertial effect by moving in the direction opposite to the movement of the corresponding shading member so as to soften the shock when the aforesaid shading member comes to a stop.
In a type of shutter such as disclosed in U.S. Pat. No. 4,657,366, the rear movement shading member is retracted from the photographing image plane just before the photographing, from the state where both the front and rear shading members shade the photographing image plane. Subsequently, an exposure is taken upon traveling of the front movement shading member. In this type of shutter, the camera blurring should be prevented when the rear movement shading member is retracted from the photographing image plane.
In this case, if balancer weights were provided for both of the front and rear shading members, the entire camera body would inevitably become larger. In addition, the material used for the balance is expensive.
Also, in a camera of such a type that the rear movement shading member is retracted before the photographing, making a means for giving a greater retracting thrust to the rear movement shading member in order to allow the rear movement shading member to be retracted reliably from the photographing image plane leads to making the camera body larger.
Furthermore, in a camera of a focal plane shutter type such as disclosed in U.S. Pat. No. 4,458,998 or U.S. Pat. No. 4,657,366, an imbalance can be generated in the traveling of the front and rear movement shading members due to a defective magnetic attraction or temperature changes, causing the width of a slit formed by the front movement shading member and rear movement shading member for exposure to be narrower, and there is a danger that an irregular exposure may occur.
In addition to the aforesaid measures traditionally taken for preventing vibration, there is known a focal plane shutter device capable of operating a vibro-prevention in such a manner that one balancer is caused to move in the direction opposite to the respective directions in which the shading member on the rear traveling side is returned to release the double shading state (that is, the state where both the shading members on the rear and front traveling sides shade the photographing image plane) after the shutter has been released, and the shading member on the front traveling side is subsequently allowed to travel for giving exposure. In this respect, the balancer reciprocates by this series of operations because the directions of the returning operation of the shading member on the rear traveling side and the traveling operation of the shading member on the front traveling side are opposite to each other.
However, in a shutter of a double shading type (the so-called double-shaded shutter, that is, in a focal plane shutter for a camera, the shutter aperture is shaded both by the shading member on the front traveling side and the shading member on the rear traveling side in a state where the charging is completed before starting an exposure) as disclosed in the specification of the aforesaid U.S. Pat. No. 4,657,366, the stroke of the returning operation of the shading member on the rear traveling side and the stroke of the traveling operation of the shading member on the front traveling side differ from each other (the returning stroke of the shading member on the rear traveling side is shorter) when the shutter is arranged to return the shading member on the rear traveling side to a position immediately before the brake is applied to it while in traveling in the double-shaded state in which the charging is completed. As a result, if only one balancer is simply allowed to reciprocate with a same stroke, a problem is still encountered that sufficient vibro-preventive effects cannot be obtained both for the traveling operation of the shading member on the front traveling side and the returning operation of the shading member on the rear traveling side. (An invention for solving this problem is referred to as second invention).
If, for example, a setting is made so that the vibro-preventive effect is optimized for the traveling operation of the shading member on the front traveling side, the vibro-preventive effect is produced excessively on the returning operation of the shading member on the rear traveling side. On the contrary, if a setting is made so that the vibro-preventive effect is optimized for the returning operation of the shading member on the rear traveling side, the vibro-preventive effect on the traveling operation of the shading member on the front traveling side becomes insufficient.
Also, even if the strokes of the returning operation of the shading member on the rear traveling side and the traveling operation of the shading member on the front traveling side are the same, there are some cases that the mass of one member is different from that of the other because the configuration and the materials to be used are different for the shading member on the front traveling side and the shading member on the rear traveling side. In this case, if the vibro-preventive effect is set to be optimal for the shading member on the rear traveling side or the shading member on the front traveling side, there is still a problem that the vibro-preventive effect on the other member becomes excessive or insufficient.
An invention for solving the above-mentioned problems is referred to as first invention.